1. Field of the Invention
The present invention relates to a predictive coder for outputting data signals which have been subjected to variable-length coding.
2. Description of the Related Art
In conventionally known DPCM (Differential Pulse Code Modulation), when an image signal is subjected to predictive coding using a predictive coder and is output as variable-length codes, since there is a certain correlation between items of sampling data, it is assumed that the frequency of occurrence of the DPCM signal at "0" is a maximum, a signal having the shortest code length is assigned to the "0" value DPCM signal, and, as the frequency of occurrence of data values decreases, signals having larger code lengths are assigned to those values.
However, in this case, when the coefficient of a predictive filter which determines a predictive value is not "1", the frequency of occurrence of predictive differential signals at "0" is not a maximum. Namely, the graph of the frequencies of occurrence of the values of each predictive differential signal is not a normal distribution with its center at "0", as shown in FIG. 2.
As a result, a conventional apparatus, in which a signal having the shortest code length is given to the center point at which each predictive differenctial signal is "0", has a disadvantage with respect to a poor data compression efficiency.